Exploring High Dynamic Range Imaging: §3.3 Global Operator

Module by: Tianhe Yang, Sarah McGee, Taylor Johnson, Robert Ortman

The term “global” in this case refers to operators that use one average luminance for the entire image. This algorithm was taken from Erik Reinhard’s Photographic Tone Reproduction for Digital Images. In his paper, the average luminance Lw is taken as

Figure 1:

where Lw(x,y) is the luminance in the scene, N is the number of pixels, and D is some small value to prevent you from taking the log of zero. The luminance at each pixel is then scaled by the key value a and the average luminance Lw. 1

Figure 2:

The key value of a scene is a value between zero and one that indicates whether it is very light, very dark, or around middle grey. For example, the key value of a scene of a white painted room is very close to one. However, most scenes have a wide range of brightness, so the key value is usually set to middle grey, or 0.18. 2

Finally, the luminances are scaled down to a displayable range between 0 and 1.

Figure 3:

where Lwhite is mapped to the maximum luminance in the scene. For low dynamic ranges, this will also enhance the contrast in the image. For the majority of high dynamic ranges, the algorithm preserves the contrast for low luminance areas while mapping high luminances to a displayable range. However, detail is lost in scenes that have very high dynamic ranges. 3

Figure 4: Global Operator

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This work is licensed by Tianhe Yang, Sarah McGee, Taylor Johnson, and Robert Ortman under a Creative Commons Attribution License (CC-BY 2.0).

Last edited by Taylor Johnson on Dec 25, 2006 4:17 pm US/Central.